1. Field of the Invention
This invention relates to an apparatus and method for measuring properties of fluids and in particular to meters which can be calibrated "in line" in accordance with the present invention and the methods by which they are calibrated.
Multi-phase fluids are frequently encountered in the production of oil. Typically the fluid emerging from an oil reservoir will comprise a mixture of oil, water and gas. A detailed knowledge of the physical properties of the fluid mixture as it emerges from an oil reservoir is extremely useful and thus enabling production from the reservoir to be optimised.
One method of determining the physical properties is to provide appropriate meters in line with the pipe in which the oil is produced. Each meter would be calibrated in a laboratory, before insertion in the line. The problem with this calibration is that in a laboratory it would be subject to different environmental conditions, such as electrical interference patterns, than it would experience in situ. This problem would be particularly apparent with, for example, an impedance meter which would be susceptible to stray capactiances in the calibration apparatus.
The other problem in performing this laboratory calibration is choosing the correct range over which to calibrate, because it is difficult to predict the field values. In addition, small variations in the composition of the fluid used to calibrate the meter can significantly effect the calibration for example, properties such as impedance are very sensitive to the salt concentration in any water phase in the fluid. Small variations in the salt concentration can lead to large variations in the unit impedance of the fluid. Thus it is necessary to make calibrations for a large number of salt concentrations to obtain an accurate calibration and operation in the field requires the concentration of salt in the measurement fluid to be known.
It is well known that it is difficult in practice to take a sample of a multi-phase fluid which is representative of the concentrations of the fluid phases. For example, when a sample of fluid is taken in the wellbore of an oil well at certain temperature and pressure downhole conditions, the sample is then sent to a laboratory where it is recombined to down hole conditions. However, there is no knowledge of the way in which the phases changed on the way to the surface and it is possible that extra water or gas could have been combined with the fluid on the way up. It is not possible to recalibrate the meter without moving it from its position in line and returning it to the laboratory.
2. Description of the Related Art
It has been proposed to calibrate fluid flow meters by diverting flow through a calibration zone in the meter. GB 1,151,568 and EP 0,208,045 both describe systems for calibrating gas flow meters. GB 1,151,568 describes a system in which gas flow can be diverted into a calibration zone by operation of valves such that the flow of gas into the zone of known volume can be measured. EP 0,208,045 effects calibration by diverting flow into a further pipeline having an instrumented bypass line to allow calibration of the flow through the further pipeline.
U.S. Pat. No. 3,721,121 describes an apparatus for investigating multi-phase fluids, comprising a chamber for trapping a fluid sample and means for detecting the level of the interface between the two phases after separation. The trapped sample must be representative of the multi-phase fluid and no measurement is made during separation of the phases.